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Subject[PATCH v3 098/110] security: make inode_follow_link RCU-walk aware
Date
From: NeilBrown <neilb@suse.de>

inode_follow_link now takes an inode and rcu flag as well as the
dentry.

inode is used in preference to d_backing_inode(dentry), particularly
in RCU-walk mode.

selinux_inode_follow_link() gets dentry_has_perm() and
inode_has_perm() open-coded into it so that it can call
avc_has_perm_flags() in way that is safe if LOOKUP_RCU is set.

Calling avc_has_perm_flags() with rcu_read_lock() held means
that when avc_has_perm_noaudit calls avc_compute_av(), the attempt
to rcu_read_unlock() before calling security_compute_av() will not
actually drop the RCU read-lock.

However as security_compute_av() is completely in a read_lock()ed
region, it should be safe with the RCU read-lock held.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
fs/namei.c | 5 +++--
include/linux/security.h | 12 +++++++++---
security/capability.c | 3 ++-
security/security.c | 7 ++++---
security/selinux/hooks.c | 16 ++++++++++++++--
5 files changed, 32 insertions(+), 11 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 33b655d..0fa7af2 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -881,8 +881,9 @@ const char *get_link(struct nameidata *nd)

touch_atime(&last->link);

- error = security_inode_follow_link(dentry);
- if (error)
+ error = security_inode_follow_link(dentry, inode,
+ nd->flags & LOOKUP_RCU);
+ if (unlikely(error))
return ERR_PTR(error);

nd->last_type = LAST_BIND;
diff --git a/include/linux/security.h b/include/linux/security.h
index 62a6620..52febde 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -476,6 +476,8 @@ static inline void security_free_mnt_opts(struct security_mnt_opts *opts)
* @inode_follow_link:
* Check permission to follow a symbolic link when looking up a pathname.
* @dentry contains the dentry structure for the link.
+ * @inode contains the inode, which itself is not stable in RCU-walk
+ * @rcu indicates whether we are in RCU-walk mode.
* Return 0 if permission is granted.
* @inode_permission:
* Check permission before accessing an inode. This hook is called by the
@@ -1551,7 +1553,8 @@ struct security_operations {
int (*inode_rename) (struct inode *old_dir, struct dentry *old_dentry,
struct inode *new_dir, struct dentry *new_dentry);
int (*inode_readlink) (struct dentry *dentry);
- int (*inode_follow_link) (struct dentry *dentry);
+ int (*inode_follow_link) (struct dentry *dentry, struct inode *inode,
+ bool rcu);
int (*inode_permission) (struct inode *inode, int mask);
int (*inode_setattr) (struct dentry *dentry, struct iattr *attr);
int (*inode_getattr) (const struct path *path);
@@ -1837,7 +1840,8 @@ int security_inode_rename(struct inode *old_dir, struct dentry *old_dentry,
struct inode *new_dir, struct dentry *new_dentry,
unsigned int flags);
int security_inode_readlink(struct dentry *dentry);
-int security_inode_follow_link(struct dentry *dentry);
+int security_inode_follow_link(struct dentry *dentry, struct inode *inode,
+ bool rcu);
int security_inode_permission(struct inode *inode, int mask);
int security_inode_setattr(struct dentry *dentry, struct iattr *attr);
int security_inode_getattr(const struct path *path);
@@ -2239,7 +2243,9 @@ static inline int security_inode_readlink(struct dentry *dentry)
return 0;
}

-static inline int security_inode_follow_link(struct dentry *dentry)
+static inline int security_inode_follow_link(struct dentry *dentry,
+ struct inode *inode,
+ bool rcu)
{
return 0;
}
diff --git a/security/capability.c b/security/capability.c
index fae99db..7d3f38f 100644
--- a/security/capability.c
+++ b/security/capability.c
@@ -209,7 +209,8 @@ static int cap_inode_readlink(struct dentry *dentry)
return 0;
}

-static int cap_inode_follow_link(struct dentry *dentry)
+static int cap_inode_follow_link(struct dentry *dentry, struct inode *inode,
+ bool rcu)
{
return 0;
}
diff --git a/security/security.c b/security/security.c
index d7c30b0..04c8fec 100644
--- a/security/security.c
+++ b/security/security.c
@@ -581,11 +581,12 @@ int security_inode_readlink(struct dentry *dentry)
return security_ops->inode_readlink(dentry);
}

-int security_inode_follow_link(struct dentry *dentry)
+int security_inode_follow_link(struct dentry *dentry, struct inode *inode,
+ bool rcu)
{
- if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
+ if (unlikely(IS_PRIVATE(inode)))
return 0;
- return security_ops->inode_follow_link(dentry);
+ return security_ops->inode_follow_link(dentry, inode, rcu);
}

int security_inode_permission(struct inode *inode, int mask)
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index d56a829..ffa5a64 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2861,11 +2861,23 @@ static int selinux_inode_readlink(struct dentry *dentry)
return dentry_has_perm(cred, dentry, FILE__READ);
}

-static int selinux_inode_follow_link(struct dentry *dentry)
+static int selinux_inode_follow_link(struct dentry *dentry, struct inode *inode,
+ bool rcu)
{
const struct cred *cred = current_cred();
+ struct common_audit_data ad;
+ struct inode_security_struct *isec;
+ u32 sid;

- return dentry_has_perm(cred, dentry, FILE__READ);
+ validate_creds(cred);
+
+ ad.type = LSM_AUDIT_DATA_DENTRY;
+ ad.u.dentry = dentry;
+ sid = cred_sid(cred);
+ isec = inode->i_security;
+
+ return avc_has_perm_flags(sid, isec->sid, isec->sclass, FILE__READ, &ad,
+ rcu ? MAY_NOT_BLOCK : 0);
}

static noinline int audit_inode_permission(struct inode *inode,
--
2.1.4


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